INTRODUCTION - . 
XVII 
problematical; and it has thus been deemed convenient to treat 
next in order the great extinct group of Chordate animals to which 
Prof. Cope 1 has applied the name of Ostracodermi. These pertain 
either to the Class Pisces or to some lower denomination yet to be 
determined. 
Though placed in immediate association with the Urochorda and 
Agnatha by Cope, and lately supposed to be allies of the Arachnids 
by Patten 2 , few facts can be advanced in favour of either of these 
theoretical interpretations of the group. The Arachnid theory is 
based upon a complete misapprehension of the most fundamental 
points in Ostracoderm skeletal anatomy 3 ; while the comparison of 
the dorsal opening in the cranial shield of the Asterolepidae with 
the mouth of an Ascidian, as originally made by Cope 4 5 , is already 
admitted by that author himself 3 to prove untenable. That there 
were no hard parts round the mouth and in relation to paired 
appendages capable of being preserved under the ordinary condi¬ 
tions of fossilization seems to be satisfactorily demonstrated; but 
there is no justification for any further statement that jaws, pectoral 
and pelvic arches were absent. On the other hand, a symmetrical 
paired series of lateral indentations on the visceral aspect of certain 
Ostracoderm dorsal shields (e. g., Ci/athaspis 6 ) suggests the original 
presence of well-separated gill-pouches, between which it is reason¬ 
able to infer there were supporting elements of the nature of visceral 
arches. There is a distinctly movable flap or plate at the posterior 
opening of what appears to have been a common gill-cavity outside 
these pouches in some genera (e. g., Cephalaspis and PtericJitJii/s). 
In every instance when the plate between the orbital apertures can 
be distinctly observed there is a small deep pit on its visceral 
aspect, sometimes projecting as a tubercle externally; and this 
occupies the precise position that would have been held by the 
pineal body of a vertebrate brain, had such been present. A pair 
of >-shaped impressions on the visceral aspect of the dorsal shield 
1 E. D. Cope, Amer. Nat. vol. xxiii. (1889), p. 852. 
2 W. Patten, Quart. Journ. Micro. Science, vol. xxxi. (1890), pp. 359-365, 
fig. 13. 
3 A. S. Woodward, Ann. Mag. Nat, Hist. [6] vol. ri. (1890), pp. 314-316. 
4 E. D. Cope, Amer. Nat. vol. six. (1885), p. 290. 
5 E. D. Cope, ibid. vol. xxii. (1888), p. 915. 
6 See especially the figures by Kunth, Zeitschr. deutsch. geol. Ges. vol. xxiv. 
(1872), pi. i. fig. 1 ; A. von Alth, Abh. k. k. geol. Keichsanst. vol. vii. no. 1 
(1874), pi. v. fig. 1 ; and Lankester, ‘ Cephalaspidas ’ (1868), pi. ii. fig. 11. 
PART II. & 
